The present invention relates to suspension systems for off-road equipment, such as agricultural tractors, and more particularly to such suspension systems that provide hydraulic load leveling.
Off-road equipment, such as construction and agricultural vehicles, can carry widely varying loads. When a relatively heavy load is applied to the equipment, the vehicle body is forced downward with respect to the axles supporting the wheels on which the vehicle rides. This results in compression of the suspension which can adversely affect the maneuverability of the vehicle. On the other hand, if the suspension is configured for very heavy loads, the vehicle may have an undesirable ride under light load conditions.
As a result, many vehicles have automatic load leveling systems which employ one or more hydraulic cylinders between the axle and the frame of the vehicle to ensure that the frame is maintained at the proper height above the axle. When a heavy load is applied to the frame, the drop of the frame is sensed and additional hydraulic fluid is applied to the cylinder to raise the frame the desired distance from the axle. Thereafter, when the load is removed from the vehicle the frame will rise significantly above the axle. When this occurs hydraulic fluid is applied to the opposing chamber of the cylinder to lower the frame with respect to the axle. This type of automatic hydraulic load leveling system ensures that the frame and axle will be at the desired separation regardless of the size of the load applied to the vehicle.
One of the drawbacks of this load leveling system is that the opposite chambers of the double acting cylinder have separate pressure control circuits and require high pump pressure to move the cylinder in both directions. Thus the consumption of fluid from the pump for load leveling may adversely affect the availability of fluid pressure for other functions powered by the tractor. In order to compensate for that power consumption, the pump capacity would have to be increased thus raising the cost of the hydraulic system.
Although the piston within the load leveling hydraulic cylinders moves under heavy loads, the piston does not move in response to the relatively small forces due to driving the vehicle over rough terrain. Therefore, the cylinders provide a very stiff the suspension system with negligible shock absorption. This results in a very rough ride, which can be uncomfortably for the operator.
The present system provides a hydraulic load leveling system that has a passive mode that provides shock absorption.
A hydraulic circuit controls a suspension of a vehicle having a cylinder and piston for load leveling functionality. The hydraulic circuit has a first node and a second node that is connected to a piston chamber of the cylinder. A first control valve has an inlet, for connection to a supply line for pressurized hydraulic fluid in the vehicle, and has a outlet which is coupled to the first node. A control valve assembly connects the first node to a tank return line of the vehicle. In the preferred embodiment, the control valve assembly comprises a second control valve connected to operate a pilot valve. The second control valve has an inlet for connection to the pump supply line and has an outlet. The pilot operated valve has a control port connected to the outlet of the second control valve, a first port coupled to the first node, and a second port for connection to the tank return line. This group of components provides the load leveling function where the control valves are electrically operated to raise and lower the vehicle.
The shock absorption is implemented by an accumulator coupled to the first node and two valve subcircuits. The first subcircuit includes a first check valve coupling the first node to the second node and permits fluid to flow through the first check valve only in a direction from the first node to the second node. A first subcircuit orifice is connected in parallel with the first check valve, and a first relief valve preferably is connected in parallel with the first check valve and opening when pressure at the second node is a predefined amount greater than pressure at the first node. The second subcircuit includes a second check valve coupling the second node to a port of the rod chamber wherein fluid can flow through the second check valve only in a direction from the second node to the rod chamber. A second subcircuit orifice is connected in parallel with the second check valve, and preferably a second relief valve is connected in parallel with the second check valve and opening when pressure in the rod chamber is a predefined amount greater than pressure at the piston chamber.
The second subcircuit meters the flow of hydraulic fluid between the chambers of the cylinder thereby enabling the cylinder to act as a shock absorber. Because a rod is attached to one side of the piston, one of the cylinder chambers has less volume that the other. The extra fluid required for the larger chamber is sent into and out of the accumulator as needed in response to operation of the first subcircuit.